Cry me a river of possibility: Scientists design new adaptive material inspired by tears

Tunable material system designed by Harvard team is easily adaptable for diverse applications in fuel transport, textiles, optical systems, and more

BOSTON — Imagine a tent that blocks light on a dry and sunny day, and becomes transparent and water-repellent on a dim, rainy day. Or highly precise, self-adjusting contact lenses that also clean themselves. Or pipelines that can optimize the rate of flow depending on the volume of fluid coming through them and the environmental conditions outside.

When sitting at rest, the adaptive and multifunctional material is smooth, clear, and flat; droplets of water or oil on its omniphobic surface flow freely down its surface. Stretching or bending it makes the fluid surface rougher, which confers the ability to precisely control the movement of water or oil droplets. Watch videos…

A team of researchers at the Wyss Institute at Harvard University and Harvard’s School of Engineering and Applied Sciences (SEAS) just moved these enticing notions much closer to reality by designing a new kind of adaptive material with tunable transparency and wettability features, as reported yesterday in the online version of Nature Materials.

"The beauty of this system is that it’s adaptive and multifunctional," said senior author Joanna Aizenberg, Ph.D., a Core Faculty member at the Wyss Institute and the Amy Smith Berylson Professor of Materials Science at SEAS.

The new material was inspired by dynamic, self-restoring systems in Nature, such as the liquid film that coats your eyes. Individual tears join up to form a dynamic liquid film with an obviously significant optical function that maintains clarity, while keeping the eye moist, protecting it against dust and bacteria, and helping to transport away any wastes — doing all of this and more in literally the blink of an eye.

The bioinspired material is a continuous liquid film that coats, and is infused in, an elastic porous substrate — which is what makes it so versatile. It is based on a core concept: any deformation of the substrate — such as stretching, poking, or swelling — changes the size of the pores, which causes the liquid surface to change its shape.

The new bioinspired, adaptive material developed by Joanna Aizenberg and her team is a continuous liquid film that coats, and is infused in, an elastic porous substrate. Any deformation of the substrate — such as stretching — changes the size of the pores, which causes the liquid surface to change its shape. To date, the research team has demonstrated the ability to dynamically control two key functions with great precision: transparency and wettability. As shown here, when stretched, the material confers the ability to reversibly "pin" droplets of water — stopping them in their tracks.

With this design architecture in place, the team has thus far demonstrated the ability to dynamically control — with great precision — two key functions: transparency and wettability, said Xi Yao, Ph.D, Wyss Institute and SEAS postdoctoral fellow, and lead author of the study.

Sitting at rest, the material is smooth, clear and flat; droplets of water or oil on its surface flow freely off of the material. Stretching the material makes the fluid surface rougher, Yao explained. The rough surface makes it opaque for one thing, and enables one to do something never possible before: It offers the ability to make every droplet of oil or water that is placed on it reversibly start and stop in their tracks. This capability is far superior to the "switchable wettability" of other adaptive materials that exist today, Yao said, which simply switch between two states — from hydrophobic (water-hating) to hydrophilic (water-loving).

"In addition to transparency and wettability, we can fine-tune basically anything that would respond to a change in surface topography, such as adhesive or anti-fouling behavior," Yao said. They can also design the porous elastic solid such that it responds dynamically to temperature, light, magnetic or electric fields, chemical signals, pressure, or other environmental conditions, he said.

The material is a next generation of a materials platform that Aizenberg pioneered a few years ago called SLIPS. SLIPS stands for Slippery Liquid-Infused Porous Surfaces, and is a coating that repels just about anything with which it comes into contact — from oil to water and blood.

Top: Schematic showing the design of the liquid-infused dynamic material. The bottom two photographs show the dry and lubricated elastic substrates (transparent when at rest).

But whereas SLIPS is a liquid-infused rigid porous surface, "the new material is a liquid-infused elastic porous surface, which is what allows for the fine control over so many adaptive responses above and beyond its ability to repel a wide range of substances. A whole range of surface properties can now be tuned, or switched on and off on demand, through stimulus-induced deformation of the elastic material," Aizenberg said.

"This sophisticated new class of adaptive materials being designed by the Institute’s Adaptive Materials Technologies platform led by Joanna Aizenberg have the potential to be game-changers in everything from oil and gas pipelines, to microfluidic and optical systems, building design and construction, textiles, and more," said Wyss Founding Director Donald Ingber, M.D., Ph.D.

The work was supported by the Air Force Office of Scientific Research (AFOSR), the Office of Naval Research (ONR) and the Wyss Institute for Biologically Inspired Engineering at Harvard University.

In addition to Yao and Aizenberg, the paper’s coauthors included Wyss Core Faculty member L. Mahadevan, Ph.D., who is also the Lola England de Valpine Professor of Applied Mathematics at SEAS, and Professor of Organismic and Evolutionary Biology and Professor of Physics at Harvard University; Wyss Institute and SEAS Postdoctoral Fellow Yuhang Hu, Ph.D.; SEAS Staff Scientist Alison Grinthal, Ph.D.; and Tak-Sing Wong, Ph.D., formerly a Postdoctoral Fellow at the Wyss Institute and now an Assistant Professor at The Pennsylvania State University.

About the Wyss Institute for Biologically Inspired Engineering at Harvard University
The Wyss Institute for Biologically Inspired Engineering at Harvard University (http://wyss.harvard.edu) uses Nature’s design principles to develop bioinspired materials and devices that will transform medicine and create a more sustainable world. Working as an alliance among Harvard’s Schools of Medicine, Engineering, and Arts & Sciences, and in partnership with Beth Israel Deaconess Medical Center, Brigham and Women’s Hospital, Boston Children’s Hospital, Dana Farber Cancer Institute, Massachusetts General Hospital, the University of Massachusetts Medical School, Spaulding Rehabilitation Hospital, Boston University and Tufts University, the Institute crosses disciplinary and institutional barriers to engage in high-risk research that leads to transformative technological breakthroughs. By emulating Nature’s principles, Wyss researchers are developing innovative new engineering solutions for healthcare, energy, architecture, robotics, and manufacturing. These technologies are translated into commercial products and therapies through collaborations with clinical investigators, corporate alliances, and new start-ups. The Wyss Institute recently won the prestigious Webby Award, as well as the World Technology Network award for innovation in biotechnology.

About the Harvard School of Engineering and Applied Sciences
Harvard School of Engineering and Applied Sciences (SEAS) serves as the connector and integrator of Harvard’s teaching and research efforts in engineering, applied sciences, and technology. Through collaboration with researchers from all parts of Harvard, other universities, and corporate and foundational partners, we bring discovery and innovation directly to bear on improving human life and society. Alumni of note include: Steve Ballmer, A.B. ’77, co-founder and current CEO of Microsoft Corporation; Leo Beranek, Ph.D. (S.D.) ’40, HBS A.M.P. ’65, former President of BBN Technologies, National Medal of Science winner; Tony Hsieh, A.B. ’95, founder of online shoe seller Zappos.com; and Bob Metcalfe, Ph.D. ’73, University of Texas Austin, Polaris Partners, National Medal of Technology winner, co-inventor of the Ethernet. For more information, visit: http://seas.harvard.edu.